@charset "UTF-8";
body {
  min-height: 100vh;
  background: linear-gradient(120deg, #3498db, #8e44ad);
  display: flex;
  flex-direction: row;
  justify-content: center;
  align-items: center;
}

body form {
  box-sizing: border-box;
  width: 20rem;
  height: 32.36rem;
  background-color: #f1f1f1;
  padding: 4em 2.5em;
  border-radius: 1em;
}

body form h1 {
  text-align: center;
  margin-bottom: 2em;
}

body form section {
  border-bottom: solid #adadad;
  margin: 2em 0;
  position: relative;
  background: none;
}

body form section input {
  font-size: 1em;
  color: #333;
  border: none;
  width: 100%;
  height: 3em;
  outline: none;
  background: none;
  padding: 0 .25em;
}

body form section span::before {
  content: attr(data-placeholder);
  position: absolute;
  top: 50%;
  left: .25em;
  color: #adadad;
  transform: translateY(-50%);
  z-index: -1;
  transition: .5s;
}

body form section span::after {
  content: '';
  width: 0;
  height: 2px;
  position: absolute;
  background: linear-gradient(120deg, #3498db, #8e44ad);
  color: #adadad;
  transition: .5s;
}

body form section .focus + span::before {
  top: -5px;
  z-index: 0;
}

body form section .focus + span::after {
  width: 100%;
}

body form #btn提交 {
  display: block;
  width: 100%;
  height: 4em;
  border: none;
  outline: none;
  background-image: linear-gradient(120deg, #3498db, #8e44ad, #3498db);
  background-size: 200%;
  color: white;
  cursor: pointer;
  font-size: 1em;
  font-weight: bold;
}

body form #btn提交:hover {
  transition: .5s;
  background-position: right;
}

body form footer {
  margin-top: 4em;
  text-align: center;
  font-size: 1em;
}
